Suction is Key to Diving Beetle's Loving Embrace

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When it comes to wooing a female, male diving beetles don't have
it easy: A male must latch onto his mate underwater, as she
wriggles about erratically like a tiny bucking bronco.

During courtship, male diving
beetles (Dytiscidae) use special adhesive hairs, or
setae, to keep a grip on the hardened forewings, or elytra, of
females. But not all suckers are created equal — circular,
suction-cuplike hairs adhere better than spatula-shaped hairs, a
new study finds.

The "coevolution of male setae and female elytra has attracted
much attention since [Charles] Darwin," the researchers wrote in
a study detailed today (June 10) in the Journal of the Royal
Society Interface. [ Animal
Sex: 7 Tales of Naughty Acts in the Wild ]

Female beetles have rough surfaces on their forewings, which
Darwin thought helped the males to mount them. But more recent
research suggests that the rough surface may actually make it
harder for the males to stick on.

The continuous evolution of female forewings and male sucker
attachments "may instead suggest sexual conflict and an arms race
between the two sexes," the researchers wrote.

In the study, researchers from Taiwan compared the
underwater attachment ability of different types of male
diving beetles: Eretes sticticus,Hydaticus
vittatus,Hydaticus pacificus and Cybister
rugosus. The first three species have circular
suction-cup-shaped hairs, while the latter has spatula-shaped
hairs.

Those with spatula hairs have less total contact area to support
their body weight while courting a female, so the researchers
wanted to know whether these insects could attach to females as
well as the ones with suction-cup hairs.

To measure the amount of attachment force of the different hairs,
the researchers collected hairs from the insects and attached
them to glass cover slips. They used a sensitive scale to
measure the vertical suction force and specialized sensors to
measure the horizontal shearing force.

Suction-cup hairs and spatula hairs adhered with about the same
force, the researchers found. However, when the researchers took
into account body size, the suction-cup hairs provided a better
grip on the surface than spatula hairs, and were more resistant
to shearing forces like the ones male beetles might experience as
females twist and turn.